Electronic oven protection circuit



Nov. 19, 1968 c. LfANDERSON 3,412,227

ELECTRONIC OVEN PROTECTION CIRCUIT Filed Nov 18, 1965 INVENTOR CARL L. ANDERSON dam/224 9 6.0

ATTORNEYS United States Patent 3,412,227 ELECTRONIC OVEN PROTECTION CIRCUIT Carl L. Anderson, Shiloh, Ohio, assignor to The Tappan Company, Mansfield, Ohio, a corporation of Ohio Filed Nov. 18, 1965, Ser. No. 508,539 5 Claims. (Cl. 219-1055) ABSTRACT OF THE DISCLOSURE A no-load electronic oven protection circuit for controlling the application of power to the high frequency oscillator tube wherein the power is transmitted to the cavity by means of a waveguide, consisting of an enclosure coupled with the waveguide by means of common apertures and containing energy absorbent material at one end and a neon bulb sensor and photocell transducer at the other end for varying the energiza-tion current of the power tube control relay as a function of reflected energy.

This invention relates to a protective circuit or system for an electronic oven having a power tube source of high frequency energy, such as a microwave oven designed for cooking.

It is a primary object of the invention to provide by such circuit automatic protection against operation of the power tube without a load in the oven to absorb the energy, the absence of food for example in the oven while the energy is supplied causing a degree of reflection of the energy potentially harmful to the power tube.

Another object is to provide such protection circuit means including a directional coupler in association with an energy supply waveguide, with the coupler serving to detect a no-load condition in the oven and producing a responsive signal for cutting off the energy.

It is also an object of the present invention to provide protective circuit means including a directional coupler as aforesaid with means for adjusting the sensitivity of the coupler.

Other objects and advantages of the present invention will become apparent as the following description proceeds.

To the accomplishment of the foregoing and related ends the invention, then, comprises the features hereinafter fully described and particularly pointed out in the claims, the following description and the annexed drawing setting forth in detail a certain illustrative embodiment of the invention, this being indicative, however, of but one of the various ways in which the principle of the invention may be employed.

In said annexed drawing, the single figure is a diagrammatic representation of a protection circuit in accordance with the present invention.

Now referring to the drawing in detail, the numeral generally designates an electronic oven of the commonly known type used for cooking purposes, it being understood that such an oven is basically a metallic enclosure provided with a door for insertion and removal of the food to be cooked. This particular oven is shown as having a top wall 11 in which there is an opening 12 for supply of the high frequency energy, with such energy being produced by a microwave oscillator such as the magnetron designated generally by reference numeral 13.

The magnetron 13 is conventional and comprises, for example, an evacuated metal envelope 14, a plurality of radial anode vanes 15, and a central cathode 16. The microwave energy is transmitted to a hollow metal waveguide 17 by a coaxial line 18, and the energy thus generated is delivered by the waveguide to the interior of the oven through the top wall opening 12.

The envelope or anode 14 of the oscillator is connected by a wire 19 to the positive terminal 20 of a direct current source and cathode 16 is connected through contacts 21 of a normally closed relay 22 to the negative terminal 23 of such source. The operating winding of the relay is connected at one end through a rectifier 24 and resistor 25 to one terminal 26 of an alternating current supply, and the other end of the relay winding is connected to the other terminal 27 of such supply through a resistor 28 and a photoelectric cell 29. The relay circuit also includes a shunt capacitor 30 and a small variable resistor 31 in parallel with the photoelectric cell 29, with this variable resistor being operative to bleed or pass current sufficient to hold the relay 22 in its normally closed condition.

A directional coupler designated generally by reference numeral 32 is physically associated with and coupled in a particular manner to the waveguide 17, this coupler being shown as an elongated closed metal enclosure disposed against and extending along the top of the guide. The coupling is accomplished by two apertures or openings 33 and 34 in the top wall 35 of the guide 17 providing communication with the interior of the coupler. These openings are spaced longitudinally apart a distance approximately equal to one-fourth of the Wavelength of the high frequency energy generated by the microwave oscil lator and transmitted through the waveguide. A body 36 of a material absorptive of the energy is disposed in the forward end of the coupler 32, with any material which thus is the equivalent of a microwave load suitable for the intended absorption as will be more fully described. A first coupling loop section 37 extends from an interior point on the coupler 32 externally to one electrode or element of a cold cathode gaseous diode 38, such as a common neon bulb, and the other element of the diode is connected by a second coupling loop section 39 to another point on the coupler, with these points of coupler connection being of course selected to provide a large enough loop to pick up enough energy for discharge of the particular diode employed. The coupler is also provided with an adjusting screw 40 penetrating the top wall and thereby projecting variably into the interior of the coupler depending upon the degree of actuation thereof.

The arrows in full lines are intended to indicate or represent the forward high frequency energy transmitted in the operation of the oven, While the arrows in dashed lines represent energy reflected from the oven. That portion of the forward energy entering the coupler 32 is substantially fully absorbed by the load material 36, so that the coupler actually operates as a sensor of reflected energy or power. In the operation of the oven with a normal load therein, such as an article of food to be cooked, there is little reflected energy entering the coupler-sensor, with the level therein being insufllcient to cause discharge of the diode or neon bulb 38. However, if the cooking operation should be commenced without load in the oven, the reflected energy will be substantial and the level thereof within the sensor immediately effective to cause the neon bulb to discharge. Such operation of the bulb or diode is observed by the photoelectric cell 29 and the resistance of the latter decreases to provide an increased flow of current to the operating winding of the relay 22 adequate to open the contacts 21 and thereby cut off the magnetron supply.

It has been found that the adjusting screw 40 of the coupler-sensor, which is located just to the rear of the rearmost waveguide coupling opening 33, provides an adjustment of the sensitivity of the coupler according to the described mode of operation. As a more specific example, in a coupler approximately eleven inches long, the center of the screw opening in the top wall is located five and one-fourth inches from the rear wall and approximately one inch to the rear of the center of the opening 33, with the exciting probe two and one-half inches to the rear of the centerline of opening 33, The configuration is preferably such that, without the screw penetrating the coupler, the relatively small amount of energy refiected with a normal load in the oven is effective to produce discharge of the neon bulb 38 and the screw 40 is then advanced into the coupler until the bulb is extinguished. The response of the coupler has been made very sensitive with this manner of adjustment of the elfective level of the reflected energy, and the protection being automatically immediately effective in the event of an excess of the reflected energy. While the theory of this sensitivity adjustment is not fully understood, it is believed that the impedance characteristic of the coupler is altered with the result of some combining of forwardly directed energy with the reflected energy for such establishment of the control level. The screw does, in any event, provide the disclosed biasing of the neon bulb and is the preferred expedient for the purpose.

Other modes of applying the principle of the invention may be employed, change being made as regards the details described, provided the features stated in any of the following claims or the equivalent of such be employed.

I, therefore, particularly point out and distinctly claim as my invention:

1. A protection system for an electronic oven and the like having a source of high frequency energy, controllable means for supplying an operating voltage to said source, a waveguide through which the energy is transmitted from the source to the oven, a chamber coupled to said waveguide for receiving both forward and reflected high frequency energy, means in said chamber for absorbing a portion of the forward energy, sensor means in said chamber for detecting oven-reflected energy, and control means for cutting off the supply of the operating voltage to the source in response to detection by said sensor means of reflected energy in the waveguide in excess of a predetermined value.

2. A protection system as set forth in claim 1 wherein said chamber comprises an elongated housing adjacent said waveguide and having apertures in the central portion thereof for communication with said waveguide, said absorbing means being located in one end of said housing and subject primarily to forward directed energy and said sensor means being located in the other end of said housing and subject primarily to reflected energy whereby said sensor is responsive to an increase of reflected energy above a predetermined level.

3. A protection system as set forth in claim 2 wherein said sensor means comprises a neon bulb mounted in said housing for receipt of high frequency energy therein, said neon bulb being ignited when suificient reflected energy above a predetermined value is received to provide a signal for said control means.

4. A protection system for an electronic oven and the like having a source of high frequency energy, controllable means for supplying an operating voltage to said 0 source, a waveguide through which the energy is transmitted from the source to the oven, directional sensor means coupled to the waveguide for detecting oven-reflected energy therein, said directional sensor means receiving portions of forward and reflected energy in the guide and including means for absorption of a portion of the forward energy, and control means including an electrical discharge device coupled to the sensor means to respond to the reflected energy therein, for cutting off the supply of operating voltage to the source in response to detection by said sensor means of reflected energy in the waveguide in excess of a predetermined value.

5. A protection system as set forth in claim 4, including tuning means adjustably penetrating the sensor means for biasing of said discharge device.

References Cited UNITED STATES PATENTS 2,704,802 3/1955 Blass et a1. 21910.55 3,175,165 3/1965 Dayhotf 331-66 3,281,567 10/1966 Meissner et a1 2l9-10.77

OTHER REFERENCES Siemens, German application 1,148,677, printed May RICHARD M. WOOD, Primary Examiner.

L. H. BENDER, Assistant Examiner. 

